Molecular hybridization studies with Mason-Pfizer virus (MPV) 70S RNA demonstrated that approximately 20% of the MPV genome is endogenous to rhesus monkeys and these sequences are highly conserved in other Old World monkeys. An additional 40% of the MPV genome could be detected in rhesus and other Old World monkeys by relaxing the hybridization conditions. Squirrel monkey retrovirus, (SMRV) another type-D retrovirus, was shown to be endogenous for that species. MPV and langur virus (LV) have identical electrophoretic patterns in gels and could not be distinguished in RIAs for their 27,000 d major internal proteins. Their reactivity was slightly different in an RIA for SMRV p36 and markedly different in RIAs for MPV p10-12 or LV p10-12. Interspecies RIAs have also been established between LV and MPV and between SMRV and MPV. The sequences of the RNA genome of the MMTVs for C3H and RIII mice that are transmitted via a nongerm line mechanism have been isolated and shown to be present as a provirus in the DNA of mammary tumors that arise early in life, but not in normal tissue of C3H and RIII. None of these sequences were detected in the mammary tumors of low or moderate incidence strains.